The invention relates generally to disc drives and specifically to disc drive seek controllers. More particularly, the invention relates to sine seek controllers operating within temperature or voltage margins.
Disc drives are used in workstations, laptops and personal computers to store large amounts of information in a readily accessible form. Typically, a disc drive includes a magnetic disc which is rotated at a constant high speed by a spindle motor. The disc surfaces are divided into a series of concentric data tracks that can store information as magnetic transitions on the disc surface.
A disc drive also includes a set of magnetic transducers that are used to either sense existing magnetic transitions during a read operation or to create new magnetic transitions during a write operation. Each magnetic transducer is mounted in a head. Each head is mounted to a rotary actuator arm via a flexible element which can accommodate movement of the head during operation. The actuator arm serves to selectively position the head over a particular data track to either read data from the disc or to write data to the disc.
The actuator arm is driven by a voice coil motor. The magnetic transducers, mounted in the heads, are present at the ends of the arms which extend radially outward from a substantially cylindrical actuator body. This actuator body is moveably supported by a ball bearing assembly known as a pivot bearing or pivot bearing assembly. The actuator body is parallel with the axis of rotation of the discs. The magnetic transducers, therefore, move in a plane parallel to the discs surface.
The voice coil motor typically includes a coil which is mounted in the actuator arm at the end opposite the heads. This coil is permanently immersed in a magnetic field resulting from an array of permanent magnets which are mounted to the disc drive housing. Application of current to the coil creates an electromagnetic field which interacts with the permanent magnetic field, causing the coil to move relative to the permanent magnets. The voice coil motor essentially converts electric current into mechanical torque. As the coil moves, the actuator arm also moves, causing the heads to move radially across the disc surface.
Control of this movement is accomplished via a servo system. In this control system, position (or servo) information is prerecorded on at least one surface of one of the discs. The servo system may be dedicated, which means that an entire disc surface is prerecorded with servo information. In this case, a particular head is dedicated to reading only servo information. Alternatively, the servo system can be embedded meaning that the servo information is interweaved with the user data, and is intermittently read by the same heads which are used to read and write information.
Servo systems typically include two controllers, a seek controller and a tracking controller. The seek controller manages large head movements for approximate placement of the actuator arm. Then, the tracking controller is responsible for the small displacements necessary to follow a particular track.
High performance disc drive customers require a drive""s operation to be verified over a wide range of conditions, such as hot and cold temperatures and high and low voltages. In addition, the environments in which drives are implemented may often demonstrate similar conditions. These conditions greatly affect physical characteristics of drive components, thus changing drive operation. High temperatures and low voltages, for instance, tend to degrade performance and can lead to permanent drive damage, directly or indirectly. Therefore, it is necessary to design disc drives such that they can be operated in a wide range of temperature margins.
One of the main problems with drive operation over a wide range of temperatures is servo coil resistance variation. The higher the temperature, the higher the resistance, and vice versa. Since the servo controller is designed to seek from one position to another in a minimum amount of time, the controller often tends to put the coil current in saturation, i.e., the controller demand is larger than the available current due to limits on the voltage supply and coil back electromotive force, during the acceleration phase to accelerate as fast as possible. The higher the coil resistance, the lower the coil current saturation point, thus limiting the amount of acceleration and deceleration that can be achieved. When the coil current is in saturation, the actuator drive can not supply any more current, regardless of the controller""s demand. Because of this, the controller is limited in its controlling abilities and it can only decrease the acceleration. The same is true for the deceleration phase of the seek. If the coil current is in saturation, the controller can only decrease the xe2x80x9cbrakingxe2x80x9d force. This is a problem if the seek operation is moving too fast for the allowable deceleration to stop the actuator on the desired destination. This leads to significant overshoot and an increase in seek time. In addition, calibrations that are performed during the deceleration phase will produce inaccurate results under this condition.
In addition to the fluctuation in temperature margins, higher or lower nominal voltages similarly appear as lower or higher coil resistance to the servo controller, respectively. Therefore, voltage margins can also cause significant overshoot problems and inaccurate calibrations.
Because of these conditions, it has been necessary to detect sub-optimal conditions during a seek operation and adjust the controller to compensate for them. In the past, current in the acceleration saturation phase was measured and if it was lower than a nominal value, the demanded velocity signal would be scaled down by a scalar value. This scalar value, labeled SDEM (slope-demand), is calculated according to equation (1) below:
SDEM=(measured current average)/(Ixe2x80x94NOM1),xe2x80x83xe2x80x83(1)
where I_NOM is a minimal current value that is determined empirically. The higher the coil resistance or the lower supply the voltage, the lower the measured current would be. Thus, the demand velocity would be scaled down, thereby preventing the deceleration phase of the seek operation from entering saturation. Any higher than nominal conditions resulted in no change to the demand velocity signal since there is no risk of entering saturation in the deceleration phase. This was accomplished by clipping the maximum value of this scalar value to one.
However, with the introduction of sine seek controllers, i.e., a seek controller that is open-loop until the peak of the deceleration phase is reached, simply scaling down the velocity demand is not enough. While scaling down the velocity demand does help prevent deceleration saturation at low voltage and/or high temperatures, another problem is introduced. Sine seek controllers operate by looking for a constant amount of velocity error between the demand velocity and the estimated velocity to decide when to switch from acceleration to deceleration. When the velocity error is less than this fixed value, the switch is made. At that switch point, the sine shaped feed forward current transitions the controller from acceleration to deceleration. Once the feed forward sequence has completed, i.e., the peak of the deceleration phase is reached, the controller will close the loop and return to velocity control. FIG. 1 is a graph of an estimated velocity profile for a sine seek controller under nominal conditions.
It can be seen from FIG. 1 that velocity 11 is plotted along the vertical axis and tracks to go 13 is plotted along the horizontal axis. The desired position 15 on a disc, i.e., the point to where the controller is seeking to, is located at the origin. The demand velocity curve 10 is a table of velocity values at various distances from the desired position. The demand velocity curve 10 is stored in a memory (not shown) accessible to the controller.
Thus, as can be seen, at the origin, i.e., desired position, the demand velocity is zero and is at its greatest further away from the origin. The switching curve 12 is derived from the demand velocity curve by an offset equal to a constant velocity error switch point value 17.
When a seeking operation is performed, the sine seek controller injects a feed forward signal having a particular frequency into the actuator which moves the actuator assembly at a certain acceleration until an acceleration saturation point is reached as shown at point 16. At this point the velocity of the actuator still increases but not as fast. Once the velocity crosses the switch curve 12, the feed forward signal transitions the actuator from acceleration to deceleration. When the velocity crosses the demand velocity curve, velocity control for the actuator is implemented and the actuator is controlled by a velocity error signal output by the controller which is the difference between the demand velocity curve and the actual velocity signal fed back to the controller.
To prevent exciting resonances when the loop is closed, the velocity error switch point is set empirically for each seek length range such that the estimated velocity should be slightly more than the demand velocity after the feed forward sequence is completed.
At lower than nominal conditions, however, the actuator cannot accelerate as quickly thus achieving less momentum by the time the switch point is reached. Because of this, the feed forward current signal over compensates and the estimated velocity is less than the demand velocity when the loop is closed. This forces the controller to cause the actuator to accelerate again for a very short period to catch up with the demand before entering the deceleration phase. FIG. 2 is a graph of a demand velocity profile at lower than nominal conditions. The bump in the profile excites resonances which greatly affects the controller""s ability to settle on a desired destination.
Thus, a need remains for a method of controlling a sine seek controller under higher and lower than nominal conditions.
The invention involves a method of operating a sine seek controller under voltage and temperature operating margins.
According to a first aspect of the invention, there is provided a method for adaptively controlling operation of a sine seek controller of a disc drive. The sine seek controller provides a feed forward signal to an actuator during a portion of the operation of the actuator and the sine seek controller provides a velocity control signal to the actuator during another portion of the operation of the actuator. The sine seek controller also stores a velocity error switch point. The method includes steps of supplying a feed forward signal to an actuator operatively coupled to the controller, measuring a signal ouput by the controller is the amplitude of the feed forward signal is decreased by a first scalar value if the measured signal is lower than a first nominal value and the velocity error switch point is increased by a second scalar value if the measured signal is higher than a second nominal value. The second scalar value is different from the first scalar value.
According to a second aspect of the invention, there is provided a disc drive having a controller and an actuator operatively coupled to the controller. The controller supplies a feed forward signal during a portion of a seek operation and a velocity control signal during another portion of a seek operation. The controller also stores a velocity error switch point. The actuator receives the feed forward signal and velocity control signal. The controller is programmed to detect the feed forward signal output by it. The controller decreases an amplitude of the feed forward signal by a first scalar value if the detected feed forward signal is lower than a first nominal value, and increases the velocity error switch point by a second scalar value if the measured signal is higher than a second nominal value.
According to a third aspect of the invention, the method also includes decreasing the velocity error switch point by the first scalar value if the measured signal is lower than the first nominal value.